Phase-separated semiconductors containing magnetic nanostructures are relevant systems for the realization of high-density recording media. Here, the controlled strain engineering of Ga$delta$FeN layers with Fe$_y$N embedded nanocrystals (NCs) textit{via} Al$_x$Ga$_{1-x}$N buffers with different Al concentration $0<x_mathrm{Al}<41$% is presented. Through the addition of Al to the buffer, the formation of predominantly prolate-shaped $varepsilon$-Fe$_3$N NCs takes place. Already at an Al concentration $x_mathrm{Al}$,$approx$,5% the structural properties---phase, shape, orientation---as well as the spatial distribution of the embedded NCs are modified in comparison to those grown on a GaN buffer. Although the magnetic easy axis of the cubic $gamma$-Ga$_y$Fe$_{4-y}$N nanocrystals in the layer on the $x_mathrm{Al} = 0%$ buffer lies in-plane, the easy axis of the $varepsilon$-Fe$_3$N NCs in all samples with Al$_x$Ga$_{1-x}$N buffers coincides with the $[0001]$ growth direction, leading to a sizeable out-of-plane magnetic anisotropy and opening wide perspectives for perpendicular recording based on nitride-based magnetic nanocrystals.
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